Fluid-solid reaction systems, such as gas-solids reaction systems, often require the solids to be retained in early stages of the reaction system while the vapor product, essentially free of solids, is processed in downstream equipment. It is desirable in these systems that the solids be as completely removed as possible from the vapor before transferring the vapor to the downstream equipment. High solids retention in the early stages of the reaction system is desirable if the solids may contaminate the vapor product or downstream vapor process handling systems, and/or increase the capital and operating costs of downstream particulate capture devices such as wet gas scrubbers, electrostatic precipitators, or filters. Additionally, in reaction systems that use small particle catalysts, the loss of catalyst particles during operation means that additional catalyst has to be added during operation to make up for the catalyst loss. Particularly in cases where the cost of catalyst is high, even marginal improvements in solid particle retention can lead to substantial reductions in operating costs. Therefore, improvements in high efficiency solids/vapor separation systems are of particular interest.
One method for separating solids from a gas-solids flow is to pass the gas-solids flow through one or more cyclone separators. For example, cyclone separators are conventionally used to separate particles from gas-solids flows in fluidized bed reactor systems such as FCC reactors and oxygenate-to-olefin reactors. In these systems, cyclone separators can be arranged in “stages” so that the lower density or gas output of a first cyclone separator stage becomes the input for a second cyclone separator stage.
Although the cyclone separators can be arranged in stages to improve efficiency, in practice the number of stages is limited by constraints on the input and output flows of the cyclones. Once the majority of solids have been removed from a gas flow, the remaining solids in the flow may not be sufficient to allow a conventional cyclone separator to operate at full efficiency. In particular, it is difficult to design a conventional multistage cyclone separator having three or more stages, as the amount of solids in the input flow for any third or later cyclone stages is often too low for fully efficient operation of a conventional cyclone. Due to the low rate of solid catalyst particle flow, the required diameter of the final third or fourth cyclone stage dipleg in a conventional multi-stage separator at a typical design dipleg catalyst flux of 150 lb/ft2*sec (6.4 kg/m2*sec) can be a ½ inch (1.2 cm) or less. At such a dipleg diameter, the dipleg is prone to catalyst bridging or compaction. Catalyst bridging and/or compaction prevents outflow of catalyst from the cyclone stage and therefore causes poor separation efficiency. However, if a larger diameter dipleg is used it will potentially result in a very long solids residience time in the dipleg which will potentially be sufficient to defluidize the catalyst in the dipleg and thus cause the dipleg to not discharge the solids from the dipleg. Also the dipleg potentially may not seal properly at such a low flow of solids since gas could flow back into the cyclone through the dipleg, which also reduces the separation efficiency.
In spite of the above difficulties, multi-stage separators having third and/or fourth stage separators can be desirable for high cost catalysts in order to minimize catalyst losses. Additionally, the catalyst remaining in a gas flow after a second separator stage is typically rich in fine particles, which are often desirable to retain in a fluidized bed system as the fine particles improve the fluidization properties of the system.
U.S. Pat. No. 5,690,709 to Barnes describes a separator for removing particles from a gas stream. The separator can be used as a third stage separator and a fourth stage separator in a multi-stage separator for removing particles from a gas flow exiting the regenerator of an FCC reactor. The third stage separators are operated with an underflow that includes up to 2.5% of the gas entering the separators. The underflow of the third stage separator is fed into the inlet of the fourth stage separator, while an underflow of the fourth stage separator is captured in a storage vessel for eventual disposal of any collected particles. The gas output of the fourth stage separator is sent to a waste heat recovery system.
U.S. Pat. No. 6,673,133 to Sechrist et al. describes using third stage cyclones located in a separate vessel for further separating particles from a gas flow, such as the exit gas flow from a regenerator.
What is needed is an improved process and/or apparatus for removing solid particles in gas-solids reactors operating in a hydrocarbon rich vapor environment and also a process for returning the collected particles back into the reactor and/or the regenerator, such as in reaction systems that use molecular sieve type catalysts. The process and/or apparatus should allow for improved particle retention, including improved fine particle retention. The process and apparatus should also maintain or improve the efficiency of the reaction system for producing the desired reaction product. Additionally, the process and apparatus should allow the additionally retained particles to be returned back to the reactor for further use.